Differential thermal movement between the spacer frame and the glass panes is a key contributor to the aging of insulating glass edge seal and of the insulating glass unit (IGU) itself. Using finite element analysis (FEA) the authors modeled the thermal movements occurring in the edge seal of a large IGU (1.5×2.1 m²) as a result of temperature variations (-30°C to +60°C) for three commercially available spacer bars of different material and design. The model was based on nylon corner keys for the aluminum and galvanized steel spacers and bent corners for the stainless steel spacers. The nylon corner keys were assumed to be solid and firmly bonded to the spacers; whereas the bent corners were modeled as solid, bent metal corner keys, also firmly bonded to the spacers. Since actual bent corners are hollow, the model tends to overestimate the stresses for this corner design. As expected, at the low temperature, the corners are pulled inward, resulting in a bending angle >90°; while at the high temperature, the corners are pushed outwards, resulting in a bending angle <90°. Monitoring the changes occurring in the thickness of the polyisobutylene primary seal along the circumference of the IGU, the authors found that the stainless steel spacer had, by far, the least effect on the change in the cross-sectional area, while the aluminum spacer had the most substantial effect. This finding is in keeping with the expected performance based on the difference in thermal expansion coefficients between spacer material and float glass. Thus, changes in the effective cross-sectional area of the primary seal available for diffusion that arise from differential thermal movements, are likely to account for the observed performance differences of IGUs having different spacer materials.